This invention relates to early assessment of fatigue damage in an aluminum structural member by detecting fatigue-induced microcracks in a surface oxide film. More particularly, this invention relates to a gel electrode that is adapted to be pressed against a selected region of an oxidized metal surface and developed by an electrical pulse to form a visible display showing the location of fatigue-induced microcracks in the region.
Cyclic, low stress loading of an aluminum structural member produces fatigue that creates cracks in the metal and ultimately leads to catastrophic failure of the member. In the early stages of fatigue, prior even to the formation of significant cracks in the aluminum, microcracks form in the more brittle oxide film that covers the aluminum surface. It has been found that the severity of oxide cracking is related to the extent of fatigue damage in the underlying aluminum and provides a basis for predicting the useful lifetime of the structural member. Prior methods for detecting the microcracks, such as a reanodization method described in U.S. Pat. No. 4,160,702, are useful for laboratory tests, but have not been suitable for assessing damage to a structural part in the field, particularly because they require removal of the part. In addition, prior methods provide general readings over a part surface, but are not readily adaptable for selectively testing regions of the surface or for pinpointing the sites of the microcracks.
Therefore, it is an object of this invention to provide a simpler method for the early assessment of fatigue damage in a selected region of a structural member formed of a metal such as aluminum by the detection of fatigue-induced microcracks in a surface oxide film, which method is quickly carried out on the member to provide a visual display showing the number, location and size of microcracks in the region. Optionally, the method also provides an electrical measure of the severity of microcracking. The method is suitable for detecting microcracks without damage to the member or requiring removal from its structural environment and provides an accurate basis for predicting the useful life of the member prior to failure.
It is also an object of this invention to provide an electrical probe, and a method employing the probe, for mapping a selected region of an oxidized metal surface to visually display the sites of fatigue-induced microcracks. The probe comprises a pliable, nonadherent tip that is adapted to be placed in contact with the region and electrically developed to show the microcracks in a few seconds or even substantially faster. The probe is portable for readily testing an accessible surface of a structural member while in its working environment. Optionally, the probe is adapted to provide an electrical measure of the extent of oxide microcracking in addition to the visual display.